FIG. 1 shows a coordinate system to explain a conventional vehicle holding method. In the coordinate system of FIG. 1, an abscissa is a time axis t, while pressures p are indicated with the aid of an ordinate.
A vehicle is to be able to be held automatically at a standstill for a long time without an undesirable rolling away of the vehicle with the aid of the conventional vehicle holding method schematically shown in FIG. 1. For this purpose, at a point in time to, an initial holding pressure pi is built up in at least one wheel brake cylinder of a hydraulic braking system of the vehicle with the aid of at least one (motorized) brake pressure buildup device of the hydraulic braking system. In the coordinate system of FIG. 1, a graph pbrake indicates a wheel brake cylinder pressure pbrake (actually) present in the at least one wheel brake cylinder.
At a point in time t1, at least one shutoff valve situated between the at least one wheel brake cylinder and the at least one brake pressure buildup device is closed, whereby initial holding pressure pi is temporarily “enclosed” in the at least one wheel brake cylinder. In the coordinate system of FIG. 1, an actuator pressure pactuator (actually) present between the at least one shutoff valve and the at least one brake pressure buildup device is indicated with the aid of a further graph pactuator. Because of the closing of the at least one shutoff valve, the at least one brake pressure buildup device may be deactivated/shut off after point in time t1, so that actuator pressure pactuator present between the at least one shutoff valve and the at least one brake pressure buildup device is (nearly) equal to zero. In contrast, wheel brake cylinder pressure pbrake may be kept nearly at initial holding pressure pi for a longer period. To nonetheless compensate for a pressure reduction of wheel brake cylinder pressure pbrake occurring in the course of time, at a point in time t2, the at least one brake pressure buildup device is reactivated and operated in such a way that actuator pressure pactuator present between the at least one shutoff valve and the at least one brake pressure buildup device is increased at least to initial holding pressure pi. Actuator pressure pactuator present between the at least one shutoff valve and the at least one brake pressure buildup device is increased so strongly that after opening of the at least one shutoff valve (at a point in time t2+δt, where δt→0), initial holding pressure pi is again present in the at least one wheel brake cylinder. The opening of the at least one shutoff valve (at point in time t2+δt) therefore triggers an overflow of the at least one shutoff valve, whereby wheel brake cylinder pressure pbrake is again increased to initial holding pressure pi. (A “subsequent supply” for the additional increase of wheel brake cylinder pressure pbrake is therefore no longer executed after the opening of the at least one shutoff valve.)
Initial holding pressure pi is subsequently “enclosed” in the at least one wheel brake cylinder by closing the at least one shutoff valve again.